Ionization gauge control with emission responsive control of thermionic filament heating

ABSTRACT

Ionization guage control for thermionic emission vacuum devices such as ionization gauges used in measurement of gas density and the like. The control comprises an overpressure control operative during outgassing of the device and at atmospheric pressure, based on thermionic emission in the vacuum device.

nited States tent Inventor Philip C. Harvey Bedford, Mass.

Appl. No 717,814

Filed Apr. 1, 1968 Patented Apr. 27, 1971 Assignee Norton Company Newton Highlands, Mass.

llONlZATlON GAUGE CONTROL WITH EMISSION RESPONSIIVE CONTROL OF THERMIONIIC FILAMENT HEATING 10 Claims, 3 Drawing Figs.

US. Cl 1. I 315/102, 315/106, 315/107, 315/108, 315/7, 324/33 Int. Cl ..G01n 27/62, H01j 7/16 Field of Search 315/94, 106, 107, 102, 108; 313/7; 324/33 [56] References Cited UNITED STATES PATENTS 2,940,010 6/1960 Kenney 315/107 3,292,090 12/1966 Watters 328/8 3,320,474 5/1967 Watters 315/106X 3,346,769 10/1967 Sheldon... 315/106X 3,450,880 6/1969 Mook 315/107 FOREIGN PATENTS 283,504 9/1962 Netherlands 315/94 Primary Examiner-Roy Lake Assistant Examiner-C. R. Campbell Attorneys-Oliver W. Hayes and Jerry Cohen ABSTRACT: Ionization guage control for thermionic emission vacuum devices such as ionization gauges used in measurement of gas density and the like. The control comprises an overpressure control operative during outgassing of the device and at atmospheric pressure, based on thermionic emission in the vacuum device.

ION GAUGE 6 2 |49v KS 55, P F

O VOLTS 4 IO VOLTS I iL g/m .Pa tentecl April 27, 1971 3,576,465

3 Sheets-Sheet 1 Patented April 27, 1971 3 Sheets-Sheet 3 FIG. 2

IDNIZATION GAUGE CONTROL WI'III EMISSION RESPONSIVE CONTROL OF 'IIIERMIONIC FIIJAMENT IIIEA'IING The present invention relates to controls for vacuum devices with thermionic emission elements and particularly to controls for ionization gauges used at high vacuum or measuring gas density.

It is the object of the invention to provide an improved apparatus of the class described characterized by a superior degree of overpressure protection.

The invention comprises generally an improvement in a control for a thermionic emission ionization device for measurement of vacuum or the like in which the control is response to the absence of emission current to prevent filament heating and to prevent outgas heating.

The invention is described below in illustrative embodiments with references to the accompanying drawings wherein:

FIG. 1 is a circuit diagram a first embodiment of the invention and FIG. IA is a simplified version of the circuit diagram of FIG. 1 showing the elements particularly associated with control of filament power and outgassing as a function of emission current (or absence of emission current).

FIG. 2 is a circuit diagram showing a second embodiment of the invention.

FIG. I STRUCTURE (WITH REFERENCE PART TO FIG. 1A ALSO) The ionization gauge is shown at the top of FIG. I and comprises a thermionic filament F for emitting electrons, an anode EMISSION DC CIRCUIT The emission circuit can be traced from filament F of the gauge through grid G then through the power supply to the zerovolt line A, through the resistors R16 and R18 (and in some instancesRlB, switch S 1 can be closed to introduce R13 into the circuit to change the reference emission characteristics, e.g. for operation of the control with a different gauge) to the 30 volt lines B1 and back to the filament. The resistors R16 and RIS (and R13) are the largest resistor valves (i.e. .10 k) involved in their respective branch paths and therefore controlling; the paths also include as is apparent from FIG. I the components R6, Q3, R16, R48, Q; R7, R10, R8, R19; and Q4, R17. The emission current-direct current series loop, in addition to the above defined branch paths is completed through the filament and grid (electron collector) of the ionization gauge device.

FILAMENT HEATING AND CONTROL OF EMISSION The heating circuit comprises Tl, a secondary winding of a power transfonner, a thyristor CR1 with protective resistor R1 PRESSURE MEASUREMENT CIRCUIT v A pressure dependent current is developed through resistor R47 and is applied to a logarithmic element comprising the base-emitter connection of transistor 08 and to amplifier comprising transistors 06 and 07. Conventional range switch S0 is provided to shunt any of resistors R42-R46 across the amplifiers input to provide the higher resolution reading of a linear scale when desired (compared to the several decades covered by the log element).

The emission current is also taped across a I000 megohm resistor R40 in part to provide a very small fraction thereof for calibration purposes. This calibration signal is negligible when the gauge is operating.

The amplifier output is fed to a voltmeter M for display and/or control via an optical meter relay or electrical relay connected directly to the amplifier.

For calibration purposes, a flow of ion current is simulated by tapping emission current via RM and switch S2.

GRID BIAS, OUTGAS AND OUTGAS SAFETY The ion gauge grid is biassed to 149 volts by the DC power supply. A secondary winding T2 of the power transformer provides heating of the grid for outgassing the gauge -a conventional step in vacuum practice.

The prior art provides a switch in the outgas circuit. While no vacuum technician would deliberately turn on the outgas power while the gauge is at atmospheric pressure, there is a danger that this could happen inadvertently. This is prevented in the present invention by making the outgas switch KS1 responsive to flow of emission current through relay Kl. When the operator opens switch S1 to initiate outgassing, the relay KI will not operate unless emission current is flowing (because in the absence of electron current fiow from the thermionic filament to the electron collector grid of the gauge .the above defined DC series loop has an open circuit and that loop includes the filament and collector of the device and the relay R1) and this cannot take place at atmospheric pressure or partial atmospheric pressure above high vacuum level.

The presence of this safety feature makes it safer and easier to conduct outgassing simultaneously with operation of the gauge for measuring.

STARTING AND SAFETY OF STARTING The starting circuit is based on switch S3 which when closed provides a drive current (typically l-3 milliamperes in the context of this embodiment) from the zero volt line through the resistance capacitor C4 (by switching on O5) to transistors Q1 and O3 to turn on the filament control. The turning on of transistor 05 is made possible by the closure of switch S3 which short circuits the base and collector of O5 to make it a virtual diode limited to only about 0.6 volts base voltage breakage a voltage which allows transistor O1 to turn on. However, the short circuit condition is of short duration in view of the capacitor C4 in series with the switch S3. The capacitor allows about one-half second of charge-up time before it opens the short circuit to stop current flow. If an alternative means of holding the transistor 05 is not available, it will turn off.

The alternative is emission current passing through the filament control circuit. If the gauge is under safe operating conditions (i.e. high vacuum) the heated thermionic filament will emit electrons and emission current will fiow for this purpose. If the gauge is under an unsafe condition (i.e. atmospheric pressure due, for instance, to breakage of the gauge or air release) no thermionic emission will occur and no alternative current source is available to hold transistor Q5 when it reverts from diode to transistor operation.

A reset bleedoff resistor R15 discharges capacitor C4 whenever the switch S3 is released.

While transistor O5 is on, it turns on transistor O3 to run GI and thus fire the thyristor CR1 so that the filament will be heated.

The net effect of this safety feature is that the starting circuit will turn the filament heater on for a limited period and if an unsafe condition exists, it will abandon the attempt to start the gauge. In prior art gauges, the filament would stay on under comparable conditions and rapidly oxidize and/or pick up contaminants which would severely degrade the instrument.

OVERPRESSURE CIRCUIT Two distinct modes of overpressure control are provided. A first mode derives its signal from the pressure dependent ion current through R47, transistor Q8 and resistor R36. lf transistor Q4 senses an excessive ion current, it controls transistor Q5 which cuts off output transistor Q1 of the filament control circuit to stop filament heating at the gauge and hence to stop emission and ionization.

A second mode of control derives from the presence or absence of emission current per se and is sensed directly by transistor Q5 which is incorporated in the above noted emission sensing voltage divider circuit and responds to a too low emission current or absence of emission current to turn off filament.

In a preferred realization of the FiG. 1 circuit, the circuit elements are:

Q1, Q2, Q3, Q4, Q6, Q8 2N42 48 transistor 05, Q9 2N3 566 transistor Q7 U1 83 transistor Q10 2N3 053 transistor R1, R4, R5 680 ohm resistor R2, R13, R16, R13, R22 10 K resistor R3, R7 1.8 K resistor R6 22 ohm resistor R8, R11, R32, R38 220 ohm resistor R9, R34 230 ohm resistor R10 500 ohm resistor R12, R17, R23, R37, R47 R14, R 300 K resistor R19 300 ohm resistor R21, R27, R33 30 K resistor R24, R31 0-2 K resistor R25, R29, R41 0-150 ohm resistor R26, R28, R35, R39 680 ohm resistor R30 (thermistor) 10K resistor R36 1K resistor R40 1,000M resistor R42-R46 Elk-30M range resistors R48 100 ohm resistor CR1 2N444l SCR or 2N4442 SCR CR3, CR5, CR6, CR7 SZ-SOV Zener diodes CR2, CR4, CR9 1N4005 Diode CR8 52 10V zener diode C1, C2 50 microfarad, Sol Capacitor C3, C4 10 microfarad, 50V capacitor CS 0.1 microfarad, 200V capacitor C6 0.01 microfarad, 1000V capacitor C7 10 microfarad, 250V capacitor lon Gauge NRC 563 Bayard-Alpert gauge FIG. 2 STRUCTURE A second embodiment of the invention is shown in FIG. 2. The principal difference between this embodiment and the FIG. 1 embodiment is that this embodiment is a very simple logarithmic-scale only circuit and indicates the way in which the sophisticated control of the present invention can be incorporated in even a simple control circuit. The ionization gauge 10 comprises a filament l2, grid 14 which serves as both anode and outgas heater and ion collector 16. The ion current is transferred via R-67 to a logarithmic element O-l6 which produces an emitter voltage which is a logarithmic function of the ion current. This voltage is fed to an electrometer circuit EL and indicated on a meter M. The emission circuit can be traced from the gauge filament 12 through grid 14 through the power supply (schematically indicated as batteries with resultant voltage outputs) to the zero volt line, through R61, R63, R64 and R57.

The emission heating and control circuit comprises, in the bias filament heating series loop, a transformer secondary 4.7 K resistor winding T-l, a thyristor CR-12 and the filament 12. The Thyristor is controlled by the circuit comprising transistors Q- 11 and Q12 as a two stage amplifier responsive to the difference between the 30 volt reference voltage and a voltage developed by the DC emission current through R61, R63, R64 and R57 and correcting for overcurrent or undercurrent conditions by a negative feedback signal to CR-12.

The ion gauge grid 14 is biassed to volts by the power supply. A transformer secondary winding T2 provides heating of the grid for outgassing as in the FIG. 1 embodiment. The outgas switch KS1 is operated by relay K-l which is activated by moving switch 8-1 from the B position shown in the drawing to its A position. When the operator opens switch S-l to its A position, however, the relay K-l will not operate unless emission current is flowing.

Starting of the gauge is accomplished by pressing (to a closed position) the double switch S-2 (after the power supply is turned on). This provides zero volts at the base of transistor Q-M and minus 0.7 volts at the emitter of this transistor thereby allowing conduction. This in turn supplies the reference volts by activating transistor Q-13, causing the above described transistors Q-12, Q-ll and CR-12 to begin heating the thermionic filament 12. Another consequence of tapping (i.e., momentarily closing) switch 8-2 is that about 30 volts is applied, via Q-13, to a holding circuit comprising diode CR-13, C-12, R59 and R60 with a resultant voltage to the base of transistor Q-M to hold it on even though switch 5-2 is immediately released. However, the holding circuit operation is limited to about half a second by capacitor C-12. An alternate source of voltage must become available after this holding period or the heating circuit will be shut off. This alternative source of voltage becomes available if there is a sufficiently high degree of vacuum in tube 10 to allow electrons to be thermionically emitted from filament 12 for emission current to flow. Flow of emission current raises the emitter voltage of transistor Q-15. At the same time, if the pressure in the gauge tube is not too high, the rate of ionization will not be excessive and ion current passing through R-67 and Q-16 will produce a low base voltage at Q-15 to allow operation of this transistor. So long as the combined conditions of emission current and nonexcessive ion current are met, transistor Q 15 will stay on and will hold the filament heating circuit via Q-14, Q-l3, Q-12, Q-l 1, CR 12. ln case of a slight overpressure, detected by Q-l6, or extreme overpressure causing thermionic emission to stop, transistor Q-15 will be shut off and this in turn will cause filament heating to stop.

ln a preferred realization, the FIG. 2 circuit elements are:

Q-l1, Q-13, Q-15, Q-16 2N4248 transistor Q-12, Q-l4 2N3566 transistor CR-ll, CR-13 F-6 diode CR-12 2N444l S.C.R.

R-51, R-55, R-59, R-64, R-67 R-52, R-57, R-58, R-62,

R-63, R-66 680 ohm resistor R-53, R-54, R-56, R-60 30K resistor R-61 220 ohm resistor R-65 6000M resistor C-12, C-l3, C-l4 50 microfarad, 50V capacitor C-15, C-16 01 microfarad, 200V capacitor C-19 0.01 microfarad, lOOOV capacitor While only certain preferred features of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. it is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.

lclaim:

1. In an electrical control for a vacuum ionization device having as spaced electrodes therein a thermionic emission filament, electron collecting grid and ion collector in the device and filament electrical power and bias supply means in the control for heating the filament to thermionic emission levels 4.7K resistor and causing thermionically emitted electrons to flow to the grid, the improvement wherein the control comprises:

a. means in the control including said bias supply forming a portion of an electrical series loop which is completed through the space between the device filament and electron collecting grid when thermionic emission and flow of electrons from the filament to grid occurs in the device;

and

b. means for sensing the emission current which flows in said series loop and controlling said filament electrical power supply means to terminate electrical power supply for filament heating when the current sensed in said series loop indicates an absence of thermionic emission in said device.

2. The apparatus of claim 1 further comprising means for converting said collector current to an output signal varying as a logarithmic function of. said collector current and said means for cutting off filament heating in the absence of emission current being further responsive to said output signal to cut off filament heating.

3. The apparatus of claim 2 wherein said means responsive to emission current and said collector current comprise a common sensing element.

4. The apparatus of claim 3 wherein said element is a transistor with a base connection to said device collector current output signal and an emitter connection to a source of device emission current and a transistor collector connection to means for holding the filament heating means in operation.

5. The apparatus of claim 1 further comprising means to hold the filament heating means in operation, said holding means being controlled by said means responsive to emission current.

6. The apparatus of claim 5 wherein said means responsive to emission current comprises a first transistor with a base connection'to a source of operating voltage responsive to ion current, an emitter connection to a source of voltage responsive to device emission current and a transistor collector current connection to said holding means; said holding means comprising a second transistor with a base connection to said first transistor collector, an emitter connection to a circuit datum and a collector connection to said device filament heating means, connected in such a manner as to control operation of said device filament heating means.

7. The apparatus of claim 5 further comprising means for starting the filament heating for the device by turning on said second transistor.

8. The apparatus of claim 5 further comprising switching means to short circuit the collector and base of one of said first and second transistors to start filament heating.

9. The apparatus of claim 3 wherein saidswitching means comprise means for automatically limiting the duration of such short circuit switching connection.

10. The apparatus of claim 1 wherein the control comprises outgas heating means and means for operating the outgas heating means and the improvement wherein said means operating the outgas heating means are located in the filament to grid bias circuit of the control in series with connections to at least one of the filament and grid of the device so that said means are responsive to the absence of emission current to prevent outgas heating. 

1. In an electrical control for a vacuum ionization device having as spaced electrodes therein a thermionic emission filament, electron collecting grid and ion collector in the device and filament electrical power and bias supply means in the control for heating the filament to thermionic emission levels and causing thermionically emitted electrons to flow to the grid, the improvement wherein the control comprises: a. means in the control including said bias supply forming a portion of an electrical series loop which is completed through the space between the device filament and electron collecting grid when thermionic emission and flow of electrons from the filament to grid occurs in the device; and b. means for sensing the emission current which flows in said series loop and controlling said filament electrical power supply means to terminate electrical power supply for filament heating when the current sensed in said series loop indicates an absence of thermionic emission in said device.
 2. The apparatus of claim 1 further comprising means for converting said collector current to an output signal varying as a logarithmic function of said collector current and said means for cutting off filament heating in the absence of emission current being further responsive to said output signal to cut off filament heating.
 3. The apparatus of claim 2 wherein said means responsive to emission current and said collector current comprise a common sensing element.
 4. The apparatus of claim 3 wherein said element is a transistor with a base connection to said device collector current output signal and an emitter connection to a source of device emission current and a transistor collector connection to means for holding the filament heating means in operation.
 5. The apparatus of claim 1 further comprising means to hold the filament heating means in operation, said holding means being controlled by said means responsive to emission current.
 6. The apparatus of claim 5 wherein said means responsive to emission current comprises a first transistor with a base connection to a source of operating voltage responsive to ion current, an emitter connection to a source of voltage responsive to device emission current and a transistor collector current connection to said holding means; said holding means comprising a second transistor with a base connection to said first transistor collector, an emitter connection to a circuit datum and a collector connection to said device filament heating means, connected in such a manner as to control operation of said device filament heating means.
 7. The apparatus of claim 5 further comprising means for starting the filament heating for the device by turning on said second transistor.
 8. The apparatus of claim 5 further comprising switching means to short circuit the collector and base of one of said first and second transistors to start filament heating.
 9. The apparatus of claim 8 wherein said switching means comprise means for automatically limiting the duration of such short circuit switching connection.
 10. The apparatus of claim 1 wherein the control comprises outgas heating means and means for operating the outgas heating means and the improvement wherein said means operating the outgas heating means are located in the filament to grid bias circuit of the control in series with connections to at least one of the filament and grid of the device so that said means are responsive to the absence of emission current to prevent outgas heating. 